Reduction in weight of an automobile body has been promoted by using a high-strength steel sheet in order to suppress emission of carbon dioxide gas from an automobile. Further, in addition to mild steel sheets, a lot of high-strength steel sheets having a maximum tensile strength of 980 MPa or more have been used for the automobile body in order to ensure safety of passengers.
Further, there is a requirement that such a steel sheet used for a member is formed to then be attached to an automobile as a part and then even if the member receives a shock due to collision or the like, the member is not easily destroyed, and further there is also a requirement to improve also low temperature toughness in order to ensure shock resistance at a cold district in particular. This low temperature toughness is prescribed by vTrs (Charpy fracture appearance transition temperature) or the like. Therefore, it is also necessary to consider the above-described shock resistance itself of a steel product. In addition, when the steel sheet is increased in strength, plastic deformation of the steel sheet becomes difficult, while a concern for destruction becomes higher, and therefore toughness is required as an important property.
As for a method of improving toughness in a high-strength steel sheet, for example, Patent Literature 1 discloses a manufacturing method, and there is known a method of setting a martensite phase with an adjusted aspect ratio to a main phase.
Generally, it is known that the aspect ratio of martensite relies on an aspect ratio of austenite grains before transformation. That is, martensite with a large aspect ratio means martensite transformed from non-recrystallized austenite (austenite extended by rolling), and martensite with a small aspect ratio means martensite transformed from recrystallized austenite.
It is necessary to increase a finish rolling temperature to enable recrystallization of austenite, and there is a tendency that a grain diameter of austenite and furthermore a grain diameter of martensite become large. Generally, it is known that refining of a grain diameter creates an effect of improving toughness, and therefore, when the aspect ratio decreases, it is possible to decrease a factor of toughness deterioration resulting from a shape, but toughness deterioration resulting from coarsening of crystal grains is caused, so that there is a limit in improvement in low temperature toughness.
Patent Literature 1 refers to a method for producing a thick steel sheet for structural member of a large-sized industrial construction machine and the like that includes both high strength and high toughness by obtaining 3 to 18 of an aspect ratio of prior austenite grains, but the steel sheet for automobile is required to have further excellent low temperature toughness. Further, the steel sheet having grains with such an aspect ratio has anisotropy of mechanical properties, to thus have difficulty being formed into a general automobile member, resulting in that there exists a problem that the use is limited.
Patent Literature 2 discloses that ferrite grains with an aspect ratio of 2 or less are set to a main phase to thereby fabricate a high-toughness steel sheet. However, the main phase of this steel sheet is ferrite, so that it is difficult to ensure the tensile strength of 980 MPa or more.
Patent Literature 3 discloses that carbides are made to finely precipitate in ferrite having an average grain diameter set to 5 to 10 μm, to thereby improve strength and low temperature toughness of a steel sheet. According to the method described in Patent Literature 3, solid-solution Ti and/or the like in steel are/is made to precipitate as carbide, to thereby increase strength of the steel sheet. However, in order to ensure a tensile strength of 980 MPa or more, finer precipitation and denser dispersion are needed, and detailed setting of cooling conditions after finish rolling is required. Therefore, it is conceivable that the steel sheet manufactured by this method has difficulty ensuring a tensile strength of 980 MPa or more stably.
Patent Literature 4 discloses that the structure of a steel sheet is set to a single phase made of bainite phase or bainitic ferrite phase and the amount of cementite at grain boundaries is suppressed, to thereby improve low temperature toughness of the steel sheet. However, the steel sheet described in Patent Literature 4 has a tensile strength of 604 to 764 MPa, and therefore it is conceivably difficult to ensure a tensile strength of 980 MPa or more. Additionally, manufacture of a thick hot-rolled steel sheet having a sheet thickness of 8.7 mm or more is described, but no mention is made regarding a manufacturing method of a thin hot-rolled steel sheet used for an automobile steel sheet.
Patent Literature 5 discloses that when manufacturing a high-strength steel sheet having a tensile strength of 980 MPa or more, generation of a MA (martensite-austenite mixed structure) phase to be a starting point of destruction is suppressed, to thereby improve low temperature toughness. Generally, the mechanism in which the MA phase appears results from the fact that C is concentrated in austenite by some kind of cause. Thus, the steel type described in Patent Literature 5 contains fixed amounts of Ti, Nb, V, and Mo, which are carbide forming elements, to thereby capture C to suppress concentration into austenite, and thereby the generation of the MA phase is suppressed.
However, these carbide forming elements are expensive and are required to be added in large amounts, so that the steel sheet described in Patent Literature 5 is poor in economic efficiency. Additionally, in Patent Literature 5, low temperature toughness of a welding joint portion is mentioned, but no mention is made regarding low temperature toughness of a parent metal, which is important for the steel sheet for an automobile body.
As above, a high-strength steel sheet that exceeds 980 MPa has difficulty including excellent low temperature toughness simultaneously.